Oort's conjecture on direct sums of Newton polygons of curves

For $i=1,2$, let $g_i$ be positive integers, let $\xi_i$ be symmetric Newton polygons of height $2g_i$, and let $g=g_1+g_2$. Define $\xi=\xi_1\oplus\xi_2$ to be the symmetric Newton polygon of height $2g$ obtained by taking the union of the slopes of $\xi_1$ and $\xi_2$, with the multiplicity of each slope equal to the sum of its multiplicities in $\xi_1$ and $\xi_2$.

Oort's conjecture. If $\xi_i$ occurs on $\mathcal{M}_{g_i}$ for $i=1,2$, then $\xi_1\oplus\xi_2$ occurs on $\mathcal{M}_{g_1+g_2}$.

This conjecture predicts that Newton polygons realized by curves can be combined through direct sum. The paper studies some cases of the conjecture, while the general assertion remains open.